Contents

Development

Schematic diagram of the paleogeographic evolution of Avalonia, Baltica and Laurentia. (Names in German.)

Location of the Caledonian/Acadian mountain chains in the Early Devonian Epoch. Present day coastlines are shown for reference. Red lines are sutures, capitalized names are the different continents/super-terranes that joined during the Caledonian orogeny.

The Old Red Sandstone Continent in the Devonian

The early development of Avalonia is believed to have been in
volcanic arcs near a
subduction zone on the margin of
Gondwana.[1] Some material may have accreted from volcanic island arcs which formed further out in the ocean and later collided with Gondwana as a result of plate tectonic movements. The igneous activity had started by 730 million years ago and continued until around 570 million years ago, in the late
Neoproterozoic.[2]

In the early
Cambrian, the
supercontinentPannotia broke up and Avalonia drifted off northwards from Gondwana. This independent movement of Avalonia started from a latitude of about 60° South. The eastern end of Avalonia collided with
Baltica, a continental plate occupying the latitudes from about 30°S to 55°S, as Baltica slowly rotated counterclockwise towards it. This happened at the end of the
Ordovician and during the early
Silurian.

In the late
Silurian and lower
Devonian, the combined Baltica and Avalonia collided progressively, with
Laurentia, beginning with the long extremity of Avalonia which is now attached to North America. The result of this was the formation of
Euramerica. At the completion of this stage, the site of Britain was at 30°S and Nova Scotia at about 45°S. This collision is represented by the
Caledonian folding or in North America as an early phase in the
Acadian orogeny.

In the
Carboniferous, the new continent and another terrane,
Armorica which included
Iberia, drifted in from Gondwana, trapping Avalonia between it and the continent so adding Iberia/Armorica to Euramerica. This was followed up by the arrival of Gondwana. The effects of these collisions are seen in Europe as the
Variscan folding. In North America it shows as later phases of the Acadian orogeny. This was happening at around the Equator during the later
Carboniferous, forming
Pangaea with Avalonia near its centre but partially flooded by shallow sea.

Iberia was later rotated away again as the African part of Gondwana strike-slipped past it. This last movement caused the
Alpine orogeny including the raising of the
Pyrenees during the
Miocene and
Pliocene. As a result of this, part of Avalonia is now to be found on each side of the
Straits of Gibraltar.

Consequences

This map shows the positions of the rocks of Avalonia remaining in Europe. The notes on it indicate the part which collided with Baltica in the upper Ordovician and that which collided with Laurentia in the Silurian. The parts of Avalonia now in Iberia and Morocco were carried there by rotation of Iberia during the subsequent collision with Gondwana followed by separation. These rocks are by no means all at the modern surface.

Part of the British-Belgian section formed an island in the Carboniferous, affecting the disposition of coalfields; this is known by names such as the '
London-Brabant Island'. Its bulk had an effect on the geological structure between the Ardennes and the
English Midlands by influencing the subsequent crustal folding resulting from the Variscan collision.